1. Field of the Invention
This invention relates to a composition for hydrophilic treatment of aluminum or aluminum alloy and a method for hydrophilic treatment using it. More detailedly, this invention relates to a composition for hydrophilic treatment for giving excellent hydrophilicity, deodorant properties and corrosion resistance over a long period to aluminum or aluminum alloy, more detailedly aluminum or aluminum alloy materials, aluminum or aluminum alloy products, for example aluminum or aluminum alloy-made heat exchangers, etc., and a method for hydrophilic treatment of aluminum or aluminum alloy using the composition.
2. Statement of Related Art
Heretofore, many of aluminum or aluminum alloy-made heat exchangers are designed so that the areas of the heat-radiating parts and the cooling parts can be as large as possible so as to heighten heat radiation or cooling effect, and therefor, the intervals among the fins are extremely narrow. Further, notches called louvres are made among the fins so as to make draft resistance as small as possible.
Thus, when such a heat exchanger is used for cooling, moisture in the air aggregates on the surface of the heat exchanger, particularly among the fins, the aggregated water is liable to be water droplets in proportion as the hydrophobicity of the fin surface increases, and the water droplets often cause clogging at the fin gaps to increase draft resistance and lower heat exchange rate.
Further, the water droplets which collected at the fin gaps tend to scatter by the blower of the heat exchanger, and it often occurs that the pan installed at the lower part of the heat exchanger cannot receive all the scattered water droplets and the neighborhood of the heat exchanger is soiled with water.
Therefore, in order to prevent water droplets from remaining at the fin gaps and causing clogging, some treatments for giving hydrophilicity to the surface of the aluminum or the alloy thereof and enhancing water wettability have been proposed and put into practice.
Heretofore, agents for hydrophilic treatment applied on the fin surfaces of aluminum or aluminum alloy-made heat exchangers are roughly classified into two kinds, namely those wherein organic macromolecules are bound or added to inorganic compounds, especially alkali silicates, and those composed of organic macromolecules alone. The former ones have better hydrophilicity and hydrophilicity persistency than the latter ones, and are more often used on the market. However in recent years, the problem of offensive odor generated from heat exchangers has been highlighted, and its cause is supposed to be due to scattering of the inorganic compounds.
As to usual aluminum or aluminum alloy-made heat exchangers for car air conditioners, an aluminum or aluminum alloy coil material is cut, punching molded and welding processed, and then assembled into aluminum or aluminum alloy-made heat exchangers. Then, the assembled heat exchangers are subjected, after a proper pre-washing, to a chemical conversion treatment for giving corrosion resistance such as chromic acid chromate treatment, and then subjected to a hydrophilic treatment as above-mentioned.
The reason that the chemical conversion treatment for giving corrosion resistance is carried out before the hydrophilic treatment is that hydrophilic treatment, as above-mentioned, alone cannot give corrosion resistance to aluminum or aluminum alloy-made heat exchangers.
However, it is troublesome to carry out corrosion resistance and hydrophilic treatments separately, and moreover, a washing step and a draining step such as air blow generally become necessary after the chemical conversion treatment, and therefore, steps further increase and the equipment becomes larger and more complicated. Further, as to chromic acid chromates and phosphoric acid chromates generally used for chemical conversion treatments, hexavalent chromium harmful to human bodies is contained in the treating agents, and therefore, there is the possibility that they have a bad influence on the environment and waste water treatment.
Various methods have been proposed for solving these problems. For example, Japanese Published Unexamined Patent Application No. 318496/1988 discloses "a method for surface treating heat exchangers which comprises giving hydrophilicity, corrosion resistance, antibacterial properties and antifungal properties by one surface treatment step" for omitting chemical conversion treatment. However, constituents therefor are not explicitly described even in examples as well as in claims, and are poor in specificity.
Japanese Published Unexamined Patent Application No. 171684/1988 discloses "a method for applying an aqueous agent for hydrophilic treatment, capable of giving corrosion resistance". Therein, film giving corrosion resistance and hydrophilicity is formed, on aluminum or aluminum alloy materials, by a resin synthesized from particular monomers. However, hydrophilicity given by this method is not yet enough, and in many cases, the method is generally used only for forming a substrate for hydrophilic films.
Further, Japanese Published Unexamined Patent Application No. 270977/1989 discloses "a method for hydrophilic treatment of aluminum or aluminum alloy, which gives both good hydrophilicity and good corrosion resistance". Therein, film giving corrosion resistance and hydrophilicity is formed from a particular polymer polymer P1, a polymer P2 having a particular functional group and a crosslinking agent. However, hydrophilic film formed by this method contains hexavalent chromium, which is environmentally undesirable.
Thus, a composition for hydrophilicity-giving treatment, not containing hexavalent chromium, which can form uniform film having hydrophilicity persistent over a long period and good corrosion resistance without substrate film, by directly applying it on the surface of aluminum or aluminum alloy products such as aluminum or aluminum alloy-made heat exchangers, has not yet been developed.